In electrorefining (ER) and electrowinning (EW) electrodes are immersed in an electrolyte and an electric current is passed between them. The anode is made positive and the cathode made negative so that an electric current passes through the electrolyte from anode to cathode.
In electrorefining (ER), the metal anode is soluble. That is to say that the metal enters into the electrolyte under the influence of the potential between the anode and cathode. For example, in the electrorefining of copper, the anode is made of impure metallic copper and copper ions enter the electrolyte from the anode. The copper ions, now in the electrolyte, are transported through or by the electrolyte to the cathode where they are deposited. The cathode may be of the same metal as the metal that is being deposited or it may be of a different metal. For example, in the electrorefining of copper it was at one time common to employ a cathode made of copper. However, a stainless steel permanent cathode is now commonly employed which quickly becomes coated with copper and which from then on essentially performs as a copper cathode. The deposited copper is mechanically removed or stripped from the permanent cathode and the permanent cathode reused. The copper deposited on the cathode is highly pure. Impurities that were in the impure anode may dissolve into the electrolyte or fall out as a solid as the anode is dissolved and may contain useful byproducts, for example, gold. Besides copper, metals purified by ER include gold, silver, lead, cobalt, nickel, tin and other metals.
Electrowinning (EW) differs from electrorefining in that the metal sought is imported into the cells and is already contained within the electrolyte. In the example of copper, sulphuric acid is typically employed to dissolve copper from an oxide form of copper ore and the resulting liquor, after concentration, is imported into an electrowinning cell to have the copper extracted. An anode and cathode are immersed in the electrolyte and a current is passed between them, again with the anode being positive and the cathode being negative. In electrowinning, the anode is not soluble but is made of an inert material. Typically a lead alloy anode is used in the case of copper electrowinning. The cathode may be of the same metal that is being extracted from the electrolyte or it may be of a different material. For example, in the case of copper, copper cathodes may be used although stainless steel cathodes are commonly employed which quickly become coated in copper. Under of the influence of an electric current, the metal to be won leaves the electrolyte solution and is deposited in a very pure form on the cathode. The electrolyte is circulated and concentrated by this process having given up a large proportion of its metal content. Besides copper, metals obtained by electrowinning include lead, gold, silver, zinc, chromium, cobalt, manganese, aluminium and other metals. For some metals, such as aluminium, the electrolyte is a molten material rather than an aqueous solution.
As an example of the voltages and current involved, in copper refining, the cell voltage is generally about 0.3V and in copper electrowinning is about 2.0V. In both cases the cathodic current density is about 300 A/m2 and the area of each side of the cathode at present is about 1 m2. These figures differ considerably for different metals and widely varying current densities may be used for the same metal but the invention applies to the electrorefining and electrowinning of all metals.
In ER and EW the starting point is an anode juxtaposed to a cathode in an electrolyte contained in a tank. But many cathode plates and many anode plates may be used, interleaved, with all the anode plates connected in parallel and all the cathode plates connected in parallel contained within a single tank of electrolyte. Electrically this still looks like a single cell and in the industry it is therefore commonly called a cell. In the ER and EW industry, “cell” is almost universally used to mean a tank filled with anodes and cathodes in parallel. In the ER and EW industry, “tank” can mean the same as “cell”, above, or it can mean the vessel alone, depending on the context. In tankhouses cells are connected electrically in series. A typical ER tankhouse might therefore require an electrical supply of the order of 36,000 Amps at 200 Volts.
The electrical circuit representing a typical tankhouse is shown in FIG. 1. Tanks 3, each containing one cell (composed of many cathodes 1 in parallel and many anodes 2 in parallel), are connected in series. A DC voltage source 19 is connected across the series circuit to drive the desired current through the cells 3. The total current is maintained at a desired value. Ideally, the current should divide equally between the cathodes 1. In practice, there is significant variation in the resistance of each cathode-anode current path and hence there are variations in the values of the individual cathode currents. This means in practice that the metal production process operates at below optimum efficiency.
More seriously, there is sometimes disruption to the operation of part of the cell when a short circuit develops between an anode plate and a cathode plate. This is typically due to a nodule or dendrite of metal growing from a cathode plate and increasing in size until it connects with the adjacent anode plate. The nodule of metal has to be physically removed to permit normal operation to continue.
Another disruption to normal production can occur when an individual cathode or individual anode becomes disconnected from the electrical circuit. As FIG. 2 shows, the electrical connection to cathodes 1 and to anodes 2 is typically made through lugs or hanger bars 7 which project from each side of the electrodes. On the right side, the hanger bar 7 rests on a busbar 4 which forms part of the electrical circuit. The disconnection is typically caused by corrosion or burning of the contact point 6 or by a foreign obstacle becoming jammed between the hanger bar 7 and the busbar 4 or build up of sulfate between the hanger bar 7 and the busbar. On the left side, the other hanger bar 7′ may either rest on an insulated supporting bar 4′ or this bar may be a secondary busbar, also known as an equaliser bar, so that the electrode 1 is electrically connected through two paths so as to reduce the effect of a bad contact to one of the hanger bars 4.
A short circuit results in an unusually large amount of current flowing in the cathode 1 and the anode 2 which are electrically shorted together. Methods conventionally employed to detect short circuits are less than ideal. One method is to detect the overheating of the electrodes resulting from the short circuit. This is less than satisfactory because damage to the electrode, its hanger bars 7 or the busbar 4 may have resulted due to a time delay before the short is detected.
This method will become even less acceptable as new, expensive, high-performance anodes, are introduced into electrowinning processes. In electrowinning, inert lead anodes have been commonly used. In recent years mixed metal oxide (MMO) catalytically coated titanium anodes have been increasingly adopted because of their superior properties. However, the MMO coated titanium anodes are more expensive than lead based anodes and more easily damaged by the heat generated during shorting. It has therefore become imperative that problems with the process, in particular short circuits between electrodes, are identified very quickly. Furthermore it is desirable that circumstances likely to give rise to a short circuit are identified. One indicator of an incipient short circuit is a rise in cathode or anode current above its usual value. Hence current measurement with an accuracy and resolution suitable for detecting this rise in current is a tool for identifying dangerous situations and for prompting operator action to correct the situation.
Another method of detecting shorts is to have a worker patrol the tanks using a Gaussmeter to detect the high magnetic field produced by the short-circuit current. Due to restricted labour the patrol can often be organized only once per day or a maximum of few times per day. Therefore the short may go undetected for many hours, during which time production is lost, current efficiency decreases, risk of decreased cathode quality increases and the electrodes, hanger bars and busbars may be damaged. This method has also proved very inefficient because the patrol needs to check every cell including the cells that do not have any problems. Unnecessary walking on top the cells during the patrol may also cause electrode movement and thus new short circuits. It also increases the risk of accidents. Infrared cameras are also used either by the worker patrols or in overhead cranes to detect short circuits due to heat caused by high current. The method has often proved not to give the desired results in the tankhouse environment because of the long time delay in detecting a short and also availability issues of a crane for the monitoring task.
In order to detect short circuits and bad (open) contacts there is a need to detect these problems at the level of individual cathodes or anodes by providing methods for measuring the current flowing in individual electrodes.
In prior art, U.S. Pat. No. 7,445,696 discloses an electrolytic cell current monitoring device and method, which detects not only short circuits, but open circuits as well. The apparatus comprises magnetic field sensors, e.g. Hall effect sensors, that measure magnetic field strength generated around a conductor adapted to carry electrical current to or from an electrolytic cell. The magnetic field current sensors for each cathode may be arranged on a rail car device which operates above the cells to detect the shorts and open contacts. Detection of current in all cathodes in the cell can be made simultaneously. The magnetic field sensor is brought at a distance above each electrode hanger bar aided by a capacitive proximity sensor. Further prior art is disclosed in an article “Measurement of Cathodic Currents in Equipotential Inter-Cell Bars for Copper Electrowinning and Electrorefining Plants”. Industry Applications Conference, 2007. 42nd IAS Annual Meeting. Conference Record of the 2007 IEEE; 23-27 Sep. 2007; Wiechmann, E. P., Morales, A. S.; Aqueveque, P. E.; Burgos, R. P. pp. 2074-2079, proposes a measurement technique for the cathodic currents in a dog-bone type intercell bars using linear ratiometric Hall effect sensors and ferromagnetic flux concentrators. The article discloses that cathode currents may be measured by combining the magnetic flux sensors and flux concentrators.
Prior art arrangements for measuring the cathode or anode bar currents have employed Hall effect sensors in proximity to the electrode hanger bars or interconnectors between anodes and cathodes to sense the magnetic field generated by these currents, thereby obtaining a signal proportional to the currents. However, other current carrying conductors are usually in close proximity to the Hall effect sensors and the magnetic field they produce causes inaccuracy in the current measurement. The use of pieces of magnetic material attached to the Hall effect sensor to concentrate flux through the sensor (as that disclosed in the above-mentioned article “Measurement of Cathodic Currents . . . .” by Wiechmann et al.), may also channel unwanted flux through the sensor.
In short, the problem with the prior art methods and arrangements is that they do not provide sufficiently accurate measurement results of the electric current at the point of maximum current. The maximum current occurs at the contact point where the electrode hanger bar contacts the electric busbar. Further, the known methods, which measure the current from the electrode hanger bar from a distance above or underneath the hanger bar, are very susceptible to differences in the position of the hanger bar in the direction of the busbar in relation to the position of the magnetic field sensor. Also they have proved vulnerable to significant measurement errors due to magnetic fields generated by adjacent cathodes. Therefore, the measurement accuracy obtained by prior art methods is bad and insufficient.